Teachers already have A levels. Then they do a degree in their subject areas. Often they have a masters degree in their subject area, some have a masters degree in education, these people have a distinct interest in pedagogy - the science and theory of teaching. Pedagogy is forever changing and things go in and out of fashion.
For example, I have O level English, A level English, a degree in English literature- which also covers some other humanity subjects. I also have a masters degree in English. I have a pgce. Therefore, I am well qualified to teach English at GCSE and A level. I was also a GCSE examiner and had to pass the exam board training and monitoring. This gave me an insight into the examination process and what the exam board was looking for.
With regards to subject knowledge, the exam boards do regular training sessions and provide some teaching materials. I always did a lot of extra lesson preparation and research. For example, I can quote macbeth, I have seen the play several times in the theatre and sit and quote along with the actors. Lost count of how many times I've watched it. I always played excerpts of the play in my lessons in order to consolidate learning. Ditto for just about every text that I've ever taught. All lessons have to have a plenary.
I did know a languages teacher who was teaching Chinese to the lower school. He taught himself GCSE Chinese and passed the exam with a grade 9. It would be pointless for me to take an A level English exam. I did however, write model answers and share them with students to show what was expected of them.
As regards results, I taught in college, students had target grades and I was expected to get them there. We had multi coloured spreadsheets tracking progress. All attendance, coursework assessments were on the spreadsheet. Around about February, we had to submit predicted grades to the exam boards. I was expected to know how each student would perform in the exam. At the end of August, we had a staff meeting and everyone's results were shared with the department. The results were analysed together with the examiners report. We were given targets for improvement for the upcoming academic year. Each question was analysed, together with students performance on that question.
Lesson observations covered subject knowledge and delivery of teaching. Plus, things like how engaged the students were.
CPD covered things such as subject content from the exam board. Safeguarding had to be completed on an annual basis. We also had drugs awareness, dyslexia training, autism training, lots of equality and diversity training, prevent, Google classroom training. We also had department training about the delivery of the course.
We also had to do Safeguarding training with the exam board, and, in fact, I did have to refer some Safeguarding concerns with reference to some exam answers.
I think some issues arise when a teacher is teaching outside of their subject area.
We also had a teacher who lacked the whatever to get the kids through exams. He lost coursework. He allowed kids to skip lessons. Somehow he didn't prepare them for the exam. They all failed. He was sacked.